1. Field of the Invention
The present invention relates to an organic electroluminescence device (which may also be referred to as an “organic EL device” hereinafter) and an organic laser diode, and particularly to an organic EL device and an organic laser diode that can emit light of an extremely narrow spectral width even with a lower current density.
2. Description of the Background Art
An organic semiconductor light emitting device such as an organic EL device has a simpler device structure than an inorganic semiconductor light emitting device, and can provide light of a shorter wavelength, e.g., from about 400 nm to about 550 nm so that it can have a larger capacity (higher recording density). Therefore, application to various fields has been expected, and search and development thereof have been actively conducted in recent years.
Actualization of an organic laser diode has been expected as a next-generation technology following the organic EL device. It can be considered that when the organic laser diode is actualized, a laser diode having high flexibility in design can be manufactured owing to its diversity in emission wavelength, simplicity in manufacturing process and others.
The organic laser can be roughly divided into an optically pumped organic laser and a current-pumped organic laser. In the optically pumped organic laser, optical energy (typically, irradiation with laser beams) pumps an organic material to produce excitons, and the light is emitted when the produced excitons recombine with each other. The light emission is amplified by stimulated emission, and is taken out as a laser beam. In the current-pumped organic laser, electric energy (i.e., application of a voltage across an anode and a cathode) is used for injecting holes and electrons into a laser active layer (light emitting layer). The anode injects the holes into the laser active layer (light emitting layer), and the cathode injects the electrons into the laser active layer (light emitting layer). Thereby, the holes and electrons recombine with each other in the laser active layer (light emitting layer) to emit the light, which is amplified by the stimulated emission and is taken out as the laser beam. The former has been reported in many publications and articles, but the latter is not yet actualized.
In either of the optically pumped organic laser and the current-pumped organic laser, it is required that a threshold (ASE threshold) for causing ASE (Amplified Spontaneous Emission) is sufficiently low.
T. Aimoto, Y. Kawamura, K. Goushi, H. Yamamoto, H. Sasabe, and C. Adachi, Appl. Phys. Lett. 86, 071110 (2005) has disclosed the followings 4,4′-bis-(N-carbazole)-biphenyl (CBP) thin film which is represented by the following formula (4) and is doped with 6 weight % of 4,4′-bis-[(N-carbazole)styryl]-biphenyl (which may also be referred to as “BSB-Cz” hereinafter) represented by the following formula (3) exhibits an extremely low ASE threshold of about 0.32 μJ/cm2 when nitrogen gas laser of wavelength of 337 nm (pulse width of 500 ps) is used as a pumping light source.

H. Nakanotani, C. Adachi, S. Watanabe, and R. Kato, Appl. Phys. Lett. 90, 1 (2007) has disclosed the followings. When the foregoing BSB-Cz: CBP thin film (of 500 nm in thickness) is irradiated with a cw (continuous wave) He—Cd laser beam (of 325 nm in excitation wavelength), light having an extremely narrow FWHM (Full Width at Half maximum) can be emitted with a low excitation power of about 15 W/cm2.
However, the optically pumped organic laser requires another light source for pumping a laser active material so as to obtain the laser beam, resulting in a problem of increase in size of a device and others. Therefore, it has been desired to actualize the current-pumped type of organic laser diode.
An organic EL device has been known as a light emitting device for emitting light from an organic pigment material without using a light source for pumping. The organic EL device is usually formed of an anode layer, hole injection/transport layer, light emitting layer, electron injection/transport layer and cathode layer layered in this order. When a voltage is applied across the anode and cathode layers to inject the holes and electrons into the light emitting layer, the light emitting layer emits the light. However, the light obtained from the organic EL device usually exhibits a wide angular distribution as well as a wide spectral width (low monochromaticity).
Many researches have been conducted for actualizing the laser diode by passing a large current through the organic EL device. However, this suffers from many problems such as (1) destruction or deterioration of an organic film due to heat generation with a large current density, (2) deactivation of excitons at a large current density due to interaction between excitons or interaction between excitons and polarons, (3) optical propagation loss due to light absorption by excitons, polarons and electrodes, and others. Each of these problems is remarkable in the current-pumped organic laser, and countermeasures are required for all of these problems. However, these problems are not yet overcome sufficiently.